Therefore, a direct way to eliminate the higher order temporal derivative term is to eliminate the kinetic term \(\partial _i\dot{\pi }\partial _i\dot{\pi }\) , which can be achieved by introducing the symmetry [1]}
\(\pi ^i(t,\textbf {x})\rightarrow \pi ^i(t,\textbf {x})+\xi ^i\left(t\right),\) 
